A Treatise On The Principles And Practice Of Dock Engineering

PORTLAND CEMENT. 119 Mr. Morrison * recommends the following procedure, which, he states, he has always found a safe rule :— “ Decide tentatively on quantity of large and small stones, if necessaiy trying two or three proportions. Add sand by degrees, till the mixture, after being well turned over and shaken down, shows a decided increase in bulk, at least 5 per cent; then add cement to an amount equal to between one-third and one-half of the sand, and draw up a spécification taking the amount of sand as unity.” A proportion of 2 parts of sand to 1 of cement will be found most effective for marine work, and it should be noted that the mortar made from sand and cement diminishes by one-fourth of the volume of the same materials mixed dry. The quantity of mortar should be from 10 to 15 per cent, in excess of that required to just fill the interstices. The sand should not be too fine or dust-like, and the particles should not be rendered too spherical by attrition. Hence pit sand is better than river or shore sand. The matrix is almost universally Portland cement. Hydraulic lime and Roman cement are also employed, but the range of their application is restricted. The former is useful for the foundations of buildings and the latter in cases of urgency, such as sometimes occur in tide work. Both are mucli inferior to Portland cement in strength and durability. Portland cement is an artificial product obtained by calcining clay, or shale, with chalk, or other limestone, at a high temperature. It is outside the province of the dock engineer to inquire into systems of manufacture, of which there are several, or to investigate too closely the chemical com- position of the cement he uses. Chemical analysis takes no account of the degree of calcination and fails to distinguish between free and combined lime. It certainly does become necessary to acquire some knowledge of the constituents of cement in their relation to saa-water, but this question will be considered later, and, for the present, the following may be stated as the approximate composition of an average sample of sound cement :— Lime, ................................................................ Silica............................................................ „ Alumina,...........................................................” Oxide of iron.................................................... ,, Sulphurio acid, .................................................5,, Alkalies,...................................................5,> Magnesia,............................................................. Moisture,...................................................35 ,, 100 Of the above ingredients, sulphur and magnesia are objectionable in excess of limits which, in the former case, are about 1, and in the latter, about 5 per cent. * Morrison on “ Cement Conorete,” Min. Proc. Inst. G.E., vol. cxxxix.